Synthetic corneal implants are applicable to a wide range of ophthalmic conditions. In comparison with competing treatment options, corneal implants frequently offer the advantages of relatively unobtrusive surgery, rapid patient recovery, fewer complications, and reversibility.
Polymers have become the preferred materials for the fabrication of corneal implants because they can be made to satisfy the critical requirements of biocompatibility, optical clarity, flexibility and durability in sufficiently thin films. The biocompatibility of a corneal implant is reliant on a porosity or permeability sufficient for the flow of gaseous molecules and tissue fluid components through the implant. Inadequate flow will result in corneal tissue necrosis. Hydrogel polymers may be intrinsically permeable and are used for the fabrication of certain corneal implants. Certain acrylic polymers that are not intrinsically permeable may be perforated by a focused laser beam to produce a permeable material suitable for use as a corneal implant. Irradiation of polymers with high-energy ions can generate pores along individual ion tracks thereby producing a permeable material suitable for use as a corneal implant, although ion irradiation of polymers has previously not been applied for this purpose.
The keratoprosthesis (artificial cornea) may be used as an alternative to a corneal transplant in patients affected by corneal disease or trauma. A recently approved keratoprosthesis is formed of the hydrogel poly 2-(hydroxymethacrlyate) (PHEMA) in a permeable opaque skirt bound to a non-permeable transparent pupillary region (Hicks, et. al., Prog. Retinal Eye Res. 19(2), 149-70, 2000). Corneal implants may also serve to correct or mitigate ophthalmic conditions not associated with the cornea itself. Mild myopia or nearsightedness can be corrected through implantation of intrastromal corneal rings (ICRs), which effectively flatten or decrease the curvature of the cornea. ICRs are typically made of the non-permeable acrylic poly(methylmethacrylate) (PMMA), of which intraocular lenses are also made. ICRs are in the shape of two semi-circular segments or crescents that are implanted in the cornea concentric with the pupil. The projected area of the ICRs on the cornea is sufficiently small that tissue fluid components flow around the implants and tissue necrosis does not occur. Hyperopia or farsightedness can be corrected through implantation of permeable hydrogel intracorneal lenses. The water content of hydrogels is nearly 80% by volume, which necessitates implants of this material of at least 30 microns thick to provide durability. A corneal implant may also incorporate an artificial iris, which is applicable to the treatment of several ophthalmic conditions, including replacement of a damaged iris, augmentation of an absent or incomplete iris, augmentation of an iris for the purpose of increasing opacity or changing color, and augmentation of an iris for the purpose of refractive-error compensation.
Each form of corneal implant subsumed by the present invention is designed to be implanted into the subject eye using techniques and instrumentation from the field of ophthalmic surgery, as for example described by Choyce for the placement of an artificial iris.
Certain disadvantages of previously disclosed embodiments of corneal implants are overcome by the current invention. In its most general embodiment the current invention combines controlled permeability, which is usually associated with hydrogels, with the optical clarity, flexibility, and durability of non-hydrated acrylic polymers. The corneal implants of the present invention therefore have the potential to serve as lamellar keratoprostheses whereas hydrogel implants are currently limited to full-thickness keratoprostheses. In the application of corneal reshaping the implants of the current invention are thinner than hydrogel alternatives and provide greater control over corneal reshaping than ICRs, in particular the use of asymmetric reshaping to correct for astigmatism. In the incorporation of an artificial iris in a corneal implant the current invention compares favorably with previously disclosed devices. The silicone annular mesh described by Terry and Ousley is relatively thick (around 200 microns) and has a central hole in the pupil region, which limits its placement in the cornea due to dimpling that may occur in the cornea over the hole in the pupil region and cause refractive errors. Therefore the artificial iris of Terry and Ousley must be placed deeper in the eye using a surgical procedure that requires major intrusion into the eye and a prolonged recovery period for the patient. These same disadvantages occur with the recently developed annular Dacron meshes, although these devices are ten times thinner at around 20 microns. An artificial iris has also been produced in the form of a thin graphitic annulus, which, due to brittleness, suffers from frequent fragmentation during surgical handling.